Section 1: Introduction

The first two units of this course have introduced us to the four basic forces in nature and the efforts to unify them in a single theory. This quest has already been successful in the case of the electromagnetic and weak interactions, and we have promising hints of a further unification between the strong interactions and the electroweak theory (though this is far from experimentally tested). However, bringing gravity into this unified picture has proven far more challenging, and fundamental new theoretical issues come to the forefront. To reach the ultimate goal of a "theory of everything" that combines all four forces in a single theoretical framework, we first need a workable theory of quantum gravity.

Figure 1: The fundamental units of matter may be minuscule bits of string.

As we saw in Units 2 and 3, theorists attempting to construct a quantum theory of gravity must somehow reconcile two fundamentals of physics that seem irreconcilable—Einstein's general theory of relativity and quantum mechanics. Since the 1920s, that effort has produced a growing number of approaches to understanding quantum gravity. The most prominent at present is string theory—or, to be accurate, an increasing accumulation of string theories. Deriving originally from studies of the strong nuclear force, the string concept asserts that the fundamental units of matter are not the traditional point-like particles but miniscule stretches of threadlike entities called "strings."

One of the most striking qualitative features of the string theories is that they predict the existence of extra spatial dimensions, with a total of 10 spacetime dimensions in the best-studied variants of the theory. This multitude of extra dimensions, beyond the familiar three of space plus one of time, suggests new approaches to account for some of the unsolved puzzles in the Standard Model of particle physics. String theory also has the potential to provide insights into the ultimate puzzles of cosmology, such as the nature of the Big Bang and the origin of dark matter and dark energy that we will learn more about in Units 10 and 11.

Figure 2: Above each point in our visible dimensions, a small extra-dimensional space may be hidden.

However, candidates for an acceptable framework that combines gravity with the other three fundamental forces have one characteristic that separates them, and this unit, from all of the other topics covered in this course: Quantum gravity theories in general, and string theories in particular, have virtually no connection (as yet) to experimental evidence. There is, so far, no evidence that string theory is the correct modification of Einstein's theory, which would render it compatible with quantum mechanics in our world. String theories, or at least most models that follow from string theory, are only predictive at energy scales far from what can be probed with current particle physics and cosmological observations. This is not surprising; it follows from basic dimensional analysis, which we will describe in this unit, and which suggests that we will need a great deal of luck (or a very big accelerator) to directly test any approach to quantum gravity. Enthusiasm for string theory has been based, instead, on the theoretical richness and consistency of the structure it gives rise to, as well as the fruitful connections it has enjoyed with many other areas of physics. But one should keep in mind that its proponents will eventually need to make experimental predictions that can be tested to confirm or deny the validity of the approach as a literal description of quantum gravity in our universe.

In the following sections, we will see some current frontiers of physics where the quantum properties of gravity may be visible in near-term experiments studying the interactions of elementary particles at very high energy, or the physics of the very early universe. We hope, then, to gain one or more experimental windows (albeit very indirect ones) into the world of quantum gravity.